Mechanically mutable polymer enabled by light

Human skin is a remarkable example of a biological material that displays unique mechanical characters of both soft elasticity and stretchability. However, mimicking these features has been absent in photoresponsive soft matters. Here, we present one synthetic ABA-type triblock copolymer consisting of polystyrene as end blocks and one photoresponsive azopolymer as the middle block, which is stiffness at room temperature and shows a phototunable transition to soft elastics athermally. We have synthesized an elastics we term “photoinduced soft elastomer,” where the photo-evocable soft midblock of azopolymer and the glassy polystyrene domains act as elastic matrix and physical cross-linking junctions, respectively. On the basis of the photoswitchable transformation between stiffness and elasticity at room temperature, we demonstrated precise control over nanopatterns on nonplanar substrates especially adaptable in the human skin and fabrication of packaged perovskite solar cells, enabling the simple, human-friendly, and controllable approach to be promising for mechanically adaptable soft photonic and electronic packaging applications.

The monomer M11AZC4 and the initiator (Fig. S1) were synthesized according to previous reports. (46,38) The 1 H-NMR data of the monomer M11AZC4 is shown in Fig.  S4. The triblock copolymer (TBC) PS-b-PM11AZC4-b-PS was synthesized via the atom transfer radical polymerization (ATRP) method. Synthesis of 1,1'-biphenyl-4,4'-bis(2-bromoisobutyrate) (Dibromo Initiator) 4,4'-biphenol (0.93 g, 5.00 mmol), triethylamine (1.11 g, 11.00 mmol), and THF (40 mL) were added in a 100 mL flask. Bromoisobutyryl bromide (2.53 g, 11.00 mmol) was added dropwise under stirring. The reaction was continued at RT overnight. Afterward, the mixture was poured into water, and a white solid precipitated. After filtration and recrystallization in ethanol, the white pallet crystal was obtained. 1  48 mmol) were placed in a 50-mL Schlenk flask. In a glove box, CuBr (11.6 mg, 0.081 mmol), HMTETA (18.7 mg, 0.081 mmol) and the dry anisole (5 mL) were placed in the 50-mL Schlenk flask under an N2 atmosphere. Then the sealed flask was removed from the glove box and stirred for 30 min at RT. The reaction was carried out by immersing the flask in a preheated oil bath at 85 °C. After polymerization for 24 h, the flask was immersed in liquid nitrogen. The reaction mixture was diluted with DCM (20 mL), passed through a column of alkaline alumina to remove the Cu(I) catalyst, and the product was precipitated by pouring the mixture into methanol (200 mL). It was collected by filtration and pouring into methanol. The abovementioned process is repeated three times and the obtained polymer was dried in vacuo at ambient temperature for 24 h to afford Br-PM11AZC4-Br as a yellow powder.

Synthesis of PS-b-PM11AZC4-b-PS by ATRP
The Br-PM11AZC4-Br macroinitiator (1.00 g, 0.036 mmol) was placed in a 50-mL Schlenk flask. In a glove box, CuBr (15.5 mg, 0.108 mmol), St (1.12 g, 10.8 mmol), PMDETA (18.7 mg, 0.108 mmol) and the dry anisole (1 mL) were placed in the 50-mL Schlenk flask under an N2 atmosphere. Then the sealed flask was removed from the glove box and stirred for 30 min at RT. The reaction was carried out by immersing the flask in a preheated oil bath at 90 °C. After polymerization for 24 h, the flask was immersed in liquid nitrogen. The reaction mixture was diluted with DCM (20 mL), passed through a column of alkaline alumina to remove the Cu(I) catalyst. The product was precipitated by pouring the mixture into hot methanol (400 mL) and the yellow solid polymer power was collected by filtration. And the process is repeated three times. Finally, the polymer was dried in vacuo at ambient temperature for 24 h to afford PS-b-PM11AZC4-b-PS as a yellow powder. And the polymerization process is shown in the Fig. S2.

SEC measurement Synthesis of Br-PM11AZC4-Br macroinitiator
The Br-PM11AZC4-Br macroinitiator with 56 repeat units was first prepared via ATRP. The SEC trace for the macroinitiator after reprecipitation is shown in Fig. S5.
The Br-PM11AZC4-Br was then used as a macroinitiator to synthesize azobenzene (AZ)containing TBC in a typical ATRP reaction. The growth process was confirmed by SEC, which showed a continuous decrease in elution time (Fig. S5), indicating a controlled increase in molecular weight upon chain extension from the macroinitiator Br-PM11AZC4-Br to the TBC.

The liquid crystal (LC) properties of the TBC
The LC properties of the TBC were investigated using DSC and POM. Endothermal peaks due to phase transitions at the crystal-to-LC transition (TK-LC) and LC-to-isotropic phase transition (Ti) are evident in the DSC curves (Fig. 1D); these phase transitions are also reversible. POM was used to observe the texture of the TBC. Solid samples were first heated to 180 °C and then cooled to RT. Images were captured during the cooling process. The image of the Br-PM11AZC-Br at 35 °C is shown in Fig. S8. And the POM images of the PS75-b-PM11AZC456-b-PS75 between 109 °C and 35 °C are shown in Fig. S9.

UV-Vis absorption spectra
The photoresponsive behaviors of the TBC were studied in film upon irradiation of UV light (365 nm, 100 mW/cm 2 , Fig. S12A) and then visible light (530 nm, 50 mW/cm 2 , Fig.  S12B). As shown in Fig. S12A, the spectrum initially showed an intense π-π* band in the UV region and a weak n-π* band in the visible region, indicating the existence of abundant trans AZs. The maximum absorption in film blue-shifted from 359 to 338 nm compared to that in solution in Fig. S11, attributed to the formation of H-aggregation and the parallel stacking of AZ mesogens. (47) Upon UV irradiation (365 nm) from 0 to 1.5 s, the π-π* band intensity decreased whereas the n-π* band intensity increased concomitantly, suggesting the occurrence of trans-to-cis isomerization of AZs (Fig.  S12A). Subsequent green light irradiation (530 nm) from 0 to 46 s induced the opposite cis-to-trans isomerization of AZs (Fig. S12B). The half-life period of cis isomer was measured as 5.6 h (Fig. S13), much longer than electron push-pull AZs.(48)

The stability of microphase separation (MPS) nanostructures
The MPS domains of PS blocks in TBC was designed as the physical cross-links supporting the extension of the continuous phase of PM11AZC4, which should play an important role in achieving the conversion between elasticity and stiffness. Since PS is photo-inert, we speculate that whether the UV irradiation could change MPS morphologies of the TBC. As previously reported, the regularly patterned cylindrical nanostructures in poly (ethylene oxide) (PEO)-based AZ-containing LC block copolymers could be quickly photo-directed from in-plane to out-of-plane arrangement at RT. (49) The present TBC showed nanocylinders of PS domains perpendicularly to substrate upon thermal annealing, as shown in Fig. S15A. However, no detectable changes in MPS nanostructures were observed in Figs. S15B-S15D even upon irradiation of UV light for a long time (300 s). At this case, AZs of azopolymer block should be in the cis-rich photostationary state, in which the stiff PS domains was dispersed, serving as the physical cross-links. As a result, the possible change in MPS nanostructure of the TBC can be given in Fig. S15E. Both phase transition from LC to isotropic and decreased Tg were induced in the AZ-containing block upon UV irradiation, but the PS nanocylinders formed by MPS was not influenced due to its light-inert characteristic and its Tg higher than RT, which should have great influence on mechanical properties of the TBC.

The nanopattern imprinting using TBC film
Here, the TBC dissolved in THF was spin-coated on a poly (dimethylsiloxane) (PDMS) substrate to prepare a uniform thin film, which was thermally annealed at 130 °C for 13 h. By the way, PDMS was chosen as the substrate because its flexible and transparent properties, allowing for successful attachment and bending of the TBC. Thus, it can be engineered for desired applications. Then the pre-designed PDMS grating mold with the periodicity of 2 μm was placed onto the film with an external mechanical stress (1.35 × 10 5 Pa) and UV light (600 s) applied. Subsequently, 530 nm visible light is used for photocuring, as shown in Fig. S24.                          .   Table S1.
Comparison of DMA-measured loss tangent (tan δ) of TBC and homopolymer PM11AZC4 at temperature mode. Table S3. Elasticity modulus and elongation at break of cis-rich TBC films irradiated with 530 nm light for different time.
Movie S1. Mechanical behavior of trans-rich TBC film.

Movie S2.
Mechanical behavior of cis-rich TBC film.

Movie S3.
The stretching and recovery of cis-rich TBC film.

Movie S4.
The flexible display device on the forearm.

Movie S5.
The flexible display device rotated at different angles.

Movie S6.
The naked and packaged perovskite solar cells immersed in water.